Hydrolysis of alpha-alkyl-alpha-(methylthio)methylene Meldrum's acids. A kinetic and computational investigation of steric effects.

The rates of hydrolysis of alpha-R-alpha-(methylthio)methylene Meldrum's acids (8-R with R = H, Me, Et, s-Bu, and t-Bu) were determined in basic and acidic solution in 50% DMSO-50% water (v/v) at 20 degrees C. In basic solution (KOH), nucleophilic attack to form a tetrahedral intermediate (T(OH)-) is rate limiting for all substrates (k1(OH)). In acidic solution (HCl) and at intermediate pH values (acetate buffers), water attack (k1(H2O) is rate limiting for 8-Me, 8-Et, and 8-s-Bu; the same is presumably the case for 8-t-Bu, but rates were too slow for accurate measurements at low pH.

Kinetic and thermodynamic acidities of pentacarbonyl(cyclobutenylidene)chromium complexes. Effect of antiaromaticity in the conjugate anion. An experimental and computational study.

The deprotonation of pentacarbonyl[(3-diethylamino-2,4-dimethyl)cyclobut-2-ene-1-ylidene]chromium (1d) and pentacarbonyl[(3-diethylamino-4-methyl-2-phenyl)cyclobut-2-ene-1-ylidene]chromium (1e) leads to antiaromatic conjugate anions by virtue of their being cyclobutadiene derivatives. Rate constants for the deprotonation of 1d and 1e by P2-Et and pKa values were determined in acetonitrile. Gas-phase B3LYP calculations of 1d, 1e, and their respective conjugate anions, using a generalized basis set, were also performed.

Reactions that generate aromatic molecules: is aromatic stabilization less or more advanced than bond changes at the transition state? Kinetic and thermodynamic acidities of rhenium carbene complexes.

A kinetic study of the reversible deprotonation of the rhenium carbene complexes 1H(+)(O), 1H(+)(S) and 2H(+)(O) by carboxylate ions, primary aliphatic and secondary alicyclic amines, water and OH(-) in 50% MeCN-50% water (v/v) at 25 degrees C is reported. These carbene complexes are of special interest because in their deprotonated form they represent derivatives of the aromatic heterocycles furan, thiophene and benzofuran. Intrinsic rate constants (k(o) for Delta G degrees = 0) determined from appropriate Brønsted plots for these rhenium carbene complexes and for the corresponding selenophene (1H(+)(Se)) and benzothiophene (2H(+)(S)) derivatives investigated earlier follow the orders furan < selenophene < thiophene and benzofuran less, similar benzothiophene.

Physical organic chemistry of transition metal carbene complexes. 23. Kinetic and thermodynamic acidities of cationic benzothienyl- and selenylcarbene complexes of rhenium in aqueous acetonitrile.

The pK(a) values of a cationic selenyl- (5H(+)) and a benzothienylcarbene complex (6H(+)) and rate constants for the reversible deprotonation of these complexes by water, carboxylate ions, primary aliphatic amines, secondary alicyclic amines (5H(+) only), and OH(-) (5H(+) only) were determined in 50% MeCN-50% water (v/v) at 25 degrees C. In comparison with neutral Fischer-type carbene complexes such as 1H, the cationic complexes 5H(+) and 6H(+) are much more acidic, and the intrinsic barriers to proton transfer are substantially higher. This paper discusses a variety of factors that contribute to these differences, with the most important ones being that 5H(+) and 6H(+) are cationic, which makes the C(5)H(5)(NO)(PPh(3))Re moiety a stronger pi-acceptor than the (CO)(5)M moieties, coupled with the fact that the deprotonated forms of 5H(+) and 6H(+ )are aromatic molecules.